Conventionally, messages are sent unsigned or signed. Unsigned messages cannot have their senders verified. Signed messages are verified using a central authority or a certificate authority.
The receiver of a message to be verified using a central authority receives a public key from the central authority that corresponds to the sender. Successful decryption of the message verifies the identity of the sender. The central authority holds the public and private keys of all members, and thus can send messages as if it were any member.
The receiver of a message to be verified using a certificate authority verifies the certificate with the certificate authority and extracts the verification key from the certificate.